The present invention relates to a new and improved construction of apparatus for the determination of the specific weight of selected regions of microscopic small, for instance of a size between about 5 .mu.m to 1 mm, approximately plane parallel samples.
The determination of the specific weight of selected regions of samples particles is of utmost significance for many technological fields of application, since such quite often constitutes a significant characteristic for the presence of certain properties.
In the case of gas-cooled high-temperature reactors there are employed for the retention of fission products at the fuel kernels having a size between 100 and 800 .mu.m essentially pyrocarbon layers as the enveloping or sheathing material. Pyrocarbon material can be produced so as to have different properties and different specific weights by varying the deposition parameters and the gas employed for coating at the thermal decomposition. The pyrocarbon layers which are employed with fuel particles must be capable of withstanding the different effects during the irradiation. This is possible by selecting suitable layers. Directly around the core there is applied an approximately 30 to 100 .mu.m thick porous carbon layer. The function of this carbon layer is to provide free volumes or spaces for the reception of the core swells as well as for the gaseous fission products caused by burn-off. Following this so-called "buffer layer" is one or a number of layers, specifically heavier layers formed of dense pyrocarbon, possibly also silicon carbide, having a thickness of 70 to 150 .mu.m, the function of which is, on the one hand, to prevent diffusion of the fission products out of the kernel, and, on the other hand, to form a pressure vessel for the gaseous fission products. In the last few years there have been developed a number of measuring methods and techniques for quality control of pyrocarbons, extensively rendering possible testing of the material properties.
One of the major material parameters is the specific weight. There are available for this determination a number of measuring methods, which essentially are either predicated upon the determination of the volume and related mass, or employed the technique of immersion of the layer to be measured in a liquid of known or determinable density. These values of the specific weight are always the average or mean specific weight of the layers over a number of layer fragments or also entire layers.
According to a further process there is mechanically rubbed off at a large number of particles the layer in stages of 10 to 20 .mu.m and by differential measurements there is determined the specific weight from the mass and volume change. The thus obtained profile constitute mean values derived from many similar layers. Owing to the relatively large measuring inaccurance this technique is less suitable. It does however provide additional information regarding the layer homogenity.
The value of the momentary outermost layer of the particle can be determined relatively simply and positively. Yet even here there must be taken into account that the conventionally employed immersion technique produces too high density values owing to the penetration of the immersion liquid into the open pores.
Up to the present time, due to the absence of any possibility of carrying out a density measurement with high local resolution, it is still completely unclear to what extent the density fluctuation in deposition direction influences the radiation behavior of the layer. However, from the irradiation behavior of pyrocarbon layers of different starting density, it is possible to conclude that by virtue of density fluctuations mechanical stresses are formed in the layer. Depending upon the pyrolysis gas there are made, in accordance with a dose of about 2.times.10.sup.21 EDN, final densities which attain the same values either completely independent of the starting density, at least extensively approach such, or assume even contrary values. In any event, with such changes of the specific weight there arise appreciable volume changes which endanger the stability of the coating.
From the scientific fields of biology and metallurgy there are known X-ray absorption microscopes, by means of which it is possible to determine the specific weight of selected regions of microscopic small, approximately plane parallel samples. Such microscopes, as a rule, comprise an X-ray unit and a light microscope by means of which there are illuminated by incident light illumination the sample to be examined. The region to be determined is moved into the image field of the light microscope, since as a rule the optical axis of the X-ray unit coincides with that of the light microscope. In conjunction with a standard there is taken an X-ray picture of the sample. The standard is step-like in its construction, so that different blackening of the film occurs, depending upon the step height. Since the blackening, among other things, is dependent upon the mass of the sample, it is possible with known layer thickness of the sample to draw certain conclusions regarding the specific weight, and the different blackenings allow by means of the standard an extrapolation of the specific weight of the sample.
The determination of the specific weight by means of an X-ray shadow or shadow projection microscope, wherein it is possible to determine the specific weight due to blackening of a film, apart from the complicated nature of determining the specific weight in this manner, further is associated with the drawback that between the exposure intensity and blackening there exists an additional logarithmic correlation which can be affected by the development. Hence, even with small irregularities there can arise large errors.